The DSL technology is a high-speed data transmission technology implemented through Unshielded Twisted Pair (UTP), including Asymmetrical Digital Subscriber Line (ADSL), Very-high-bit-rate Digital Subscriber Line (VDSL), Integrated Services Digital Network (ISDN)-based Digital Subscriber Line (IDSL), and Single-pair High-bit-rate Digital Subscriber Line (SHDSL).
Among various DSL technologies (xDSL), except the xDSL based on baseband transmission (for example, IDSL and SHDSL), the xDSL technologies based on passband transmission coexist with the Plain Old Telephone Service (POTS) on a twisted pair by means of the frequency division multiplexing technology, in which the xDSL occupies the high band and the POTS occupies the baseband part below 4 KHz; and the POTS signals are separated from the xDSL signals through a splitter, or combined with the xDSL signals through a combiner.
The xDSL based on passband transmission uses the Discrete Multi-Tone (DMT) modulation technology for modulation and demodulation. The system that provides multiple channels of DSL access is called “DSL Access Multiplexer (DSLAM).” The connection relations of a DSLAM system are shown in FIG. 1: The DSLAM 120 includes a customer premises transceiver unit 121 and a splitter/combiner 122. In the uplink direction, the customer premises transceiver unit 121 receives DSL signals from the computer 110, amplifies the received signals, and sends the amplified DSL signals to the splitter/combiner 122; the splitter/combiner 122 combines the DSL signals from the customer premises transceiver unit 121 and the POTS signals from the telephone terminal 130; the combined signals are transmitted through multiple Unshielded Twisted Pairs (UTPs) 140, and the splitter/combiner 151 of the peer DSLAM 150 receives the signals; the splitter/combiner 151 splits the received signals, sends the POTS signals to the Public Switched Telephone Network (PSTN) 160, and sends the DSL signals to the central office transceiver unit 152 of the DSLAM 150; and the central office transceiver unit 152 amplifies the received signals, and sends them to the Network Management System (NMS) 170. In the downlink direction, the signals are transmitted reversely.
As the band applied to the xDSL technology is higher and higher, crosstalk becomes a nuisance, especially in a high band. FIG. 2A and FIG. 2B show the Near End Cross Talk (NEXT) and the Far End Cross Talk (FEXT) in the xDSL. As shown in FIG. 2A, port 1 and port 2 in the DSLAM 210 are connected with the Remote Terminal Unit (RTU) 211 through cables. The uplink and downlink channels of the xDSL are based on the frequency division multiplexing technology, so the NEXT causes little harm to the system performance. As shown in FIG. 2B, port 1 and port 2 of DSLAM 220 are connected with the RTU 221 respectively through cables. The uplink and downlink channels of the xDSL are based on the frequency division multiplexing technology, so the FEXT deteriorates the line transmission performance drastically. For example, when xDSL services are activated for multiple users in a bundle of cables, some lines may suffer from low transmission rate and instability or even xDSL services fail to be activated because of FEXT, leading to a low activation rate of the DSLAM.
In order to achieve higher rates or greater service radiuses, the prior art uses the binding technology. The binding technology is characterized by using multiple pairs of subscriber lines concurrently as physical transmission media. At the lower band (with lower FEXT), the comprehensive performance of the binding technology is roughly the linear sum of the performance of all subscriber lines. At the higher band (with higher FEXT), the comprehensive performance of the binding technology is far less than the linear sum of the performance of all subscriber lines as affected by FEXT. Technically, the binding process simply treats the crosstalk as noise, and cannot make the most of the information transferred in the crosstalk.
In order to solve the FEXT problem in the previous binding solution, a Dynamic Spectrum Management (DSM) technology emerges recently. The DSM technology solves the FEXT problem through Multi-Input and Multi-Output (MIMO) and vectored DSL technologies on the signal plane.
With respect to the modulation mode, the MIMO technology is OFDM. As shown in FIG. 3, the conception of the OFDM is to divide a band into multiple tones of narrower frequencies, with each tone bearing a certain quantity of bits. The frequency of each tone is narrow. Therefore, the transmission function of a channel in this band is approximately regarded as a constant which verges on distortion-free transmission and facilitates processing at the receiver side. Moreover, each tone is completely orthogonal, and the tones do not affect each other.
Both the optimization of the DSM technology and the crosstalk cancellation of the MIMO technology are based on the orthogonal feature mentioned above. Generally, the receiver of each previous xDSL modem treats the interference from other modems onto this modem as noise. Therefore, the data rate accomplishable on number k tone of number n user (bkn) can be calculated through a Shannon channel capacity formula:
      b    k    n    =            log      2        (          1      +                                                                                h                k                                  n                  ,                  n                                                                    2                    ⁢                      s            k            n                                                              ∑                              m                ≠                n                                      ⁢                                                                                                  h                    k                                          n                      ,                      m                                                                                        2                            ⁢                              s                k                m                                              +                      σ            k            n                                )  
In the above formula, hkn,n is the transmission function of number n line on number k tone; hkn,m, is the crosstalk function of number m line on number k tone against number n line; σkn is the noise power of number n line on number k tone; and skn is the transmitted power of number n line on number k tone.
The above formula shows that the whole DSM rate calculation is based on each tone due to the orthogonal feature of tones. If the orthogonal feature of each tone is damaged, all DSM algorithms will change, and the algorithms enumerated above will not be applicable.
When the symbol (frame) is not synchronous between all lines, the orthogonal feature of the tone will be damaged. As shown in FIG. 4, line 1 is affected by the interference from line 2, and the symbols are not synchronous between line 1 and line 2. When line 1 performs OFDM demodulation, line 1 will handle some signals of symbol 1 and symbol 2 in line 2, which is equivalent to adding window 2 and window 3 on line 2 respectively. Evidently, window 2 and window 3 are shorter than the normal OFDM signals (as shown in window 1). Consequently, the spectrum width differs between window 2 and window 3, which damages the orthogonal feature of frequency. That is, the signals of different frequencies generate interference to each other.